1. Field of the Invention
This invention relates generally to a communications system and, more particularly, to an initial entry processor in a processing satellite of a satellite based cellular communications system.
2. Discussion of the Related Art
In a satellite based cellular communications system, a central terrestrial control processor or network operations center (NOC) generally controls one or more communications satellites operating within the communications system. Each communications satellite within the communications system services multiple users located in multiple geographic areas, known as ground cells. The communications satellites receive and transmit data signals to and from the multiple users or terrestrial terminals positioned at the different locations within the ground cells on a point-to-point manner.
In a processing satellite using time division multiple access (TDMA) on the uplink from the terrestrial terminals to the processing satellite, it is necessary to provide a means of measuring the time of arrival of transmissions from the various terrestrial terminals in a given uplink beam so that the timing of the user transmissions may be adjusted to maintain a requisite precision. This process may be resolved into two distinct phases. One directed to the initial entry and the other directed to the long term maintenance. The initial entry process is typically performed only when the terrestrial terminal is initially commissioned. In the subject processing satellite system, the location of the terrestrial terminal in geocentric Cartesian coordinates is known at the time of commissioning to within a few hundred meters, as a result of GPS (global positioning system) measurements. Also, the position of the processing satellite is known very accurately, generally to within a few meters in the same coordinate system, by maintenance of an ephemeris at the network operations center (NOC) and by dissemination of the satellite""s coordinates by means of ATM cells which are broadcast to all the terrestrial terminals through the processing satellite.
With this apriori information, the entering terrestrial terminal can target a special initial entry burst slot so that an initial entry burst (IEB) arrives at the processing satellite within about six microseconds of the true satellite timing. For an uplink transmission speed of about 500 kilosymbols per second, which is typical for the IEB, the amount of time uncertainty is equivalent to about three symbol epochs. The primary function for the initial entry processor (IEP) is to facilitate the process of refining the range precision from the uncertainty level that exists when the terrestrial terminal is being commissioned to a finer level needed for it to re-enter the system using only the ongoing synchronization maintenance provisions of the communications system. Quantitatively, this essentially amounts to shrinking the initial uncertainty at commissioning of plus/minus six microseconds to a lesser value of about plus/minus 0.25 microseconds to facilitate this re-entry.
If the initial entry procedures are not coordinated between the terrestrial terminals, there is a potential for collisions occurring between the terrestrial terminals during the initial entry process. Because of this, the initial entry processor must be able to distinguish between an initial entry burst from a single terrestrial terminal which is a valid condition and a corrupted initial entry burst from multiple terrestrial terminals which is an invalid condition. The initial entry processor must also be able to demodulate a portion of the initial entry burst to determine the identity of the commissioning user or terrestrial terminal. Moreover, the initial entry processor must function reliably under conditions wherein the signal amplitude may vary widely and where the timing clock available for forming demodulated samples cannot be altered. The initial entry processor must also be able to perform its task based on samples acquired with an arbitrary timing error. The initial entry processor must also operate with an unknown phase of the uplink signal prior to the initial entry burst arrivals.
What is needed then is an initial entry processor for a processing communications satellite that meets the above requirements. This will, in turn, provide an initial entry processor for a processing communications satellite that examines each initial entry burst slot to determine whether one and only one initial entry burst is present to provide robust collision detection, determine the timing relationship of the initial entry burst relative to the timing in the processing satellite, extract the data content of the initial entry burst to identify the terrestrial terminal, and provide information to an onboard processor in the processing satellite so that a report may be prepared and forwarded to the network operations center apprising the network operations center of the terrestrial terminals identity and the observed timing error. It is, therefore, an object of the present invention to provide a satellite based cellular communications system which utilizes an initial entry processor for a processing satellite.
In accordance with the teachings of the present invention, an initial entry processor for use in a processing satellite in a satellite based communications system is provided. The initial entry processor determines a time of arrival of an initial entry burst relative to an initial entry burst slot and identifies the particular terrestrial terminal that transmitted the initial entry burst. The time of arrival information used by the identified terrestrial terminal during subsequent communications with the processing satellite to reduce uplink timing between the terrestrial terminal and the processing satellite.
In one preferred embodiment, an initial entry processor for use in a processing satellite in a satellite based communications system includes a buffer, a detection and timing circuit, and an identity circuit. The buffer stores an initial entry burst transmitted from at least one terrestrial terminal to the processing satellite. The detection and timing circuit detects the initial entry burst and determines a time of arrival of the initial entry burst relative to the initial entry burst slot. The identity circuit determines an identity of the terrestrial terminal that transmitted the initial entry burst so that the time of arrival is used by the identified terrestrial terminal during subsequent communications with the processing satellite.
Use of the present invention provides an initial entry processor for use in a processing satellite in a satellite based communications system. As a result, the aforementioned requirements associated with initializing communication between a terrestrial terminal and a satellite have been met.